Interlock systems, which are to prevent the unauthorized starting of the motor vehicle, are installed in motor vehicles. Starting is unauthorized, e.g., if a motor vehicle is stolen or the user of the motor vehicle is under the influence of alcohol. In the latter case, the starter in a motor vehicle with an interlock system installed can be actuated only when the user had previously undergone a breath alcohol test and the value measured in the test is below a certain limit value. Since such a breath alcohol test is not intended to be a breath alcohol test performed by the police within the framework of a traffic check, attempts at deception cannot be ruled out, in which the breath alcohol test is circumvented or the motor vehicle is pushed or started by another person, even though the value measured in the breath alcohol test is above the limit value. Such attempts of deception represent a violation of the legal conditions, have far-reaching consequences for the person involved and must therefore be reliably identified or ruled out.
Prior-art interlock systems monitor, e.g., the vehicle system voltage in the motor vehicle, doing so even when the motor vehicle is stopped and the ignition is not turned on. However, a sample is requested for the breath alcohol test only after the ignition had been turned on. The drop in the vehicle system voltage during the start-up of the motor vehicle is used here as a criterion of whether or not a start has taken place. If a drop in the vehicle system voltage is indeed registered when the breath alcohol test has not been passed, an entry is made in the memory of the interlock system, which is read, e.g., by an inspection station at monthly intervals. However, the detection of the vehicle system voltage is not a reliable criterion for the question of whether the motor vehicle was started or not, because a drop in the vehicle system voltage may also be caused by turning on an air conditioner, a radio, windshield wipers or a fan in the motor vehicle.
Another possibility of demonstrating the unauthorized starting of a motor vehicle is described in U.S. Ser. No. 2002/0039951 A1. An acceleration sensor with a control means for the ignition and the fuel supply is used there on a motorcycle. The acceleration sensor also takes into account the gravitational acceleration and, in a preferred embodiment, it is able to measure the accelerations in two directions directed at right angles to one another. Signal noise and vibrations, which shall be left out of consideration, are eliminated by a downstream filter.